Bacteria Associated with Risk of Preterm Birth

Babies who are born early are at higher risk for problems with their lungs and other organs and for death. Infections in the mother’s genital tract are thought to be a major cause of preterm birth, accounting for approximately 25% to 40% of all preterm births.

Many different types of infections, caused by different combinations of micro-organisms, are apparently related to an increased risk of preterm birth. For example, a common infection called bacterial vaginosis (BV) has been associated with a sharp rise in the risk of preterm birth. However, scientists have found that although infections increase the risk of preterm birth, treating an infection does not necessarily lower the risk.

To help understand why, researchers supported by the Pregnancy and Perinatology Branch conducted a study to assess the relationship between preterm birth and selected vaginal bacteria. They collected vaginal fluid at 17 weeks’ to 22 weeks’ gestation from about 500 pregnant women who had previously had a preterm birth (and thus were at risk for another early delivery).

The researchers found that several types of bacteria—mycoplasma, mobiluncus, and atopobium—were correlated with increased risk of preterm birth. However, the extent of the increased risk was sometimes different for women of different racial and ethnic groups.

For example, the presence of mobiluncus bacteria is usually considered to indicate BV. Mobiluncus was associated with a nearly twofold increase in the risk of preterm birth for Hispanic women, but there was no association between mobiluncus and preterm birth in the other racial and ethnic groups. By contrast, another organism also associated with BV seemed to decrease, rather than increase, the risk of preterm birth for all racial/ethnic groups.

These findings help scientists understand why treating BV may not always reduce risk of preterm birth. The results suggest that focusing on specific bacterial types, rather than the infections that may result, could help scientists develop new ways to prevent preterm deliveries (PMID: 24096128).